Energy savings in mechanical pulping are highly desirable from an economical and ecological perspective and could significantly contribute to improve the overall competitiveness of the Paper and Board Industry.
One method to reduce the energy requirements of conventional thermo-mechanical pulping (TMP) without sacrificing pulp quality is low consistency (LC) refining at 70° C. to 90° C. right after the latency chest. This method typically achieves only a 5% to 8% specific energy reduction. Further, the method typically causes significant fiber shortening and loss of tear strength. Chemo-mechanical pulps (CMP) and chemo-thermo-mechanical pulps (CTMP) provide much higher potential for energy savings by LC refining because their lignin softening temperature is lower, typically below 100° C., and the more flexible fibers are more resistant to higher refining intensity and higher specific energy applications in the LC stage.
Both types of virgin fibers for papermaking, namely chemical pulps and mechanical pulps, are burdened: the first (chemical) by high wood-consumption and the second (mechanical) by high electric energy demand. The energy consumption of mechanical pulping represents about 20% of the worldwide electric energy demand of papermaking and is nearly as much as the electric energy consumed by the countries of Austria and Switzerland together.
Energy efficiency of mechanical pulping is poor as compared to other processes used in papermaking, such as pumping, drying, conveying and electric energy generation. While the energy efficiency of mechanical pulping is debated, calculations and estimations range from 0.012% to a few percent and up to 40-60%. However, it is believed that the estimation of efficiencies as high as 40-60% is in error because evidently more than 90% of the refining energy used in thermo-mechanical pulps (TMP) is converted into heat.
Less than 10% of the total energy to produce pulp for papermaking is typically used to separate the wood into individual fibers. These individual fibers are then delaminated (internal fibrillation), fines are peeled from the middle lamella and the primary and secondary layers of the fiber wall, and the remaining secondary wall is fibrillated. At the same time the fibers are flexibilized and partially even collapsed and split. These effects are achieved by many thousands of load changes (compression and relaxation). This fatiguing process in the compressible medium steam is called refining.
Methods to make the refining process more efficient have recently centered around optimizing the refining intensity, defined as the energy transfer per impact, and rapid and selective heating of the fibers beyond the lignin softening temperature in order to make them more resistant to better withstand the harsher treatment. Exposing the wood prior to refining to high compaction and sheer forces in order to create optimum separation/fracture zones in the cellulose-rich areas of the fiber wall is also one of the newer strategies. Treating wood chips with enzymes is another new development to reduce the electric energy consumption. Optimization of the refining intensity was accomplished by raising the refiner speed, by increasing the disc diameter and by proper refiner plate configuration such as nonradial expelling or turbine plates.